An exhaust system of an internal combustion engine can include a turbine wheel set in a turbine housing to create backpressure. In such a system, as the pressurized exhaust passes through the turbine housing (e.g., en route to an atmospheric outlet), the turbine wheel harnesses energy as the exhaust expands.
Various parameters may characterize a turbine wheel or a turbine housing. For example, a parameter known as “A/R” (e.g., area divided by radius) describes a geometric characteristic of a turbine housing where a smaller A/R may increase velocity of exhaust directed to a turbine wheel and provide for increased power of a turbocharger at lower engine speeds (e.g., resulting in a quicker boost rise from a compressor). However, a small A/R may also cause exhaust flow in a more tangential direction, which can reduce flow capacity of a turbine wheel and, correspondingly, tend to increase backpressure. An increase in backpressure can reduce an engine's ability to “breathe” effectively at high engine speeds, which may adversely affect peak engine power. Conversely, use of a larger A/R may lower exhaust velocity. For a turbocharger, lower exhaust velocity may delay boost rise from a compressor. For a larger A/R turbine housing, flow may be directed toward a turbine wheel in a more radial fashion, which can increase effective flow capacity of the turbine wheel and, correspondingly, result in lower backpressure. A decrease in backpressure can allow for increased engine power at higher engine speeds.
As a turbine housing and turbine wheel can create backpressure in an exhaust system, opportunities exist for exhaust leakage. For example, during operation of a turbine, a turbine housing space is at a higher pressure than its environment. Also, since exhaust gas expands across a turbine wheel, pressure downstream of the turbine wheel is considerably lower than that of a turbine housing volute region. Hence, in the foregoing example, two possible regions exist for exhaust leakage.
Exhaust leakage may be of a type that leaks out of an exhaust system to the environment or of a type that remains within an exhaust system yet bypasses a turbine wheel space. As to the latter, such leakage may occur between components of an exhaust turbine, for example, where the components may expand, contract, experience force, etc., as operational conditions vary. Further, where cycling occurs (e.g., as in vehicles), components may wear, become misaligned, etc., as cycle number increases. Whether external or internal, leakage can alter performance of a turbine wheel and turbine housing assembly. For example, a leaky turbine housing may not perform according to its specified A/R, which can complicate engine control, control of a variable geometry mechanism, etc. Various technologies and techniques described herein are directed to seals and sealing that can reduce leakage of exhaust, for example, within a turbine assembly.